The present invention relates to mobile communications arranged to transmit encoded data in accordance with a first protocol and to transmit extended data in accordance with a second protocol.
Radio transmission systems using frequency division multiplex and time division multiple access are known; an example being the GSM system for mobile telephony. A primary band between 890 megahertz and 960 megahertz is allocated for GSM transmissions with each of these 25 megahertz bands being divided into a plurality of carriers, with a 200 kilohertz displacement between said carriers. This provides for a total of 122 usable carriers within the allocated bandwidth, each divided into a total of eight transmission channels in the time domain, identified as time slot number 0 to time slot number 7. The eight time slot cycle occupies a time division multiple access (TDMA) frame of 4.615 milliseconds.
The size of finite time slots within a finite frequency band will determine the maximum theoretical data rate for a digitally transmitted signal. Standard GSM allows a sufficient data rate for encoded speech signals to be transmitted and much work has been done in terms of minimizing data rates while minimizing signal degradation.
Increasingly, cellular radio communication is being used for the transmission of data as an alternative to encoded speech, possibly as a direct modem link, a link to the Internet or as a facsimile transmission. GSM recommendations include provisions for data transmission in which, for example, the interleaving depth is increased so as to reduce the effect of errors due to fading. However, it is appreciated that a demand exists for higher rates of mobile data transmission which would require a higher bandwidth than that provided by a conventional GSM channel, or a similar channel provided within a mobile telecommunications network essentially designed for the transmission of encoded speech.
The problem of fading associated with radio transmission is well documented. In addition to causing amplitude variations, fading caused by multi-path reflections also results in frequency selective fading in which, although some frequencies are attenuated, other frequencies are enhanced due to the accumulation of in-phase reflections. The effect of these amplitude variations, in terms of signal degradation and distortion (resulting in data loss in the digital domain) are accommodated in GSM by providing interlacing and error correction. However, the problem becomes worse as signal bandwidth is increased. Under these circumstances, some parts of the signal itself, in the frequency domain, may suffer from constructive interference whereas other parts of the signal may suffer from destructive interference, possibly to the point of total extinction.
Generally, frequency components close together will suffer similar variations and may therefore be considered as being well correlated. However, others which are further apart will be less well correlated and the correlation bandwidth may be considered as the frequency separation of signals that are correlated by a particular factor, usually taken as 0.9 or better. For a narrowband signal, frequency selective distortion is usually minimized if the bandwidth is less than the correlation bandwidth of the channel. Similarly, a signal which occupies a wider bandwidth (greater than the correlation bandwidth) will be subject to more distortion. Thus, providing a greater bandwidth for a particular transmission channel will not result in a pro rata increase in the data transfer capacity. As the bandwidth increases, the introduction of errors, due to multi-path transmission, will also increase. Thus, a straightforward approach of increasing channel bandwidth to provide broadband services results in less efficient use of the available total bandwidth being made, which in turn would place severe commercial limitations upon the exploitation of broadband services of this type.
As previously stated, the total theoretical data rate is determined by the bandwidth allocation in the frequency domain and time slot duration in the time domain. Thus, if it is not possible to increase data rate by increasing bandwidth, it should be possible to increase data rate by allocating more time slots to a particular channel or to allowing each channel to be provided with the full duration of each carrier, that is to say, by removing the time division multiple access component.
The TDMA aspect of digital cellular mobile transmission is also important in terms of channel identification, power saving and signalling. Thus, if wideband access is to be given to mobile users, it is preferable for this access to be made compatible with existing schemes. Thus, in order to maintain this compatibility, it is preferable to maintain the time division multiple access component in frames similar to that used in encoded speech transmission such that, with this constraint in mind, it is then necessary to increase available frequency bandwidth within each of the allocated time slots.
A system configured to dynamically select frequencies in time slot assignments for communication with devices having different communication protocols is disclosed in United States patent publication 5,134,615. A frequency agile transmitter and receiver are combined with an adaptable time slot selector thereby enabling communications with the remote devices utilising different protocols.
The relationship between time slots and frequency channels is illustrated in FIG. 2 of the aforesaid disclosure. Three different types of TDMA devices are shown as being accommodated within each frame. In each of time slot T1 and T4, one type of device is accommodated by two time slots per frame by utilising three continuous channels CH1 to CH3. The data transmitted during these time slots is accommodated by a carrier frequency at the centre of channel CH2 with a bandwidth of three channels. In time slot T2 a second different device with less data throughput is accommodated with only one channel frequency. In time slot T3 a third different device is accommodated by utilising the bandwidth of frequency channels CH3 and CH4.
According to a first aspect of the present invention, there is provided communications apparatus for mobile telephones, arranged to transmit first encoded data in accordance with a first protocol and to transmit extended data in accordance with a second protocol, comprising first transmission means configured to transmit encoded speech data over time slots within time division multiple access frames; and second transmitting means for transmitting said extended data, wherein a second bandwidth for transmission of said extended data is divided into time slots substantially similar to said time slots for said first data which are substantially in phase with time slots for said speech data while occurring at a higher data rate than the data rate for the transmission of said speech data.
In a preferred embodiment, the first transmission means is arranged to transmit said encoded speech data in accordance with GSM recommendations.
According to a second aspect of the present invention, there is provided communications apparatus for mobile telephones, arranged to transmit a first encoded data in accordance with a first protocol and to transmit extended data in accordance with a second protocol, comprising a first transmission means for transmitting said first data over time slots within time division multiple access frames encoded in accordance with GSM recommendations; and second transmitting means for transmitting said extended data, wherein a second bandwidth for transmission of said extended data is divided into time slots substantially similar to said time slots for said first data, wherein said first transmission means and second transmitting means transmit data within frames having eight time slots.
In a preferred embodiment, the second transmitting means is arranged to transmit extended data at a second data rate higher than the data rate for the transmission of said first data.
According to a third aspect of the present invention, there is provided communications apparatus for mobile telephones, arranged to transmit first encoded data in accordance with a first protocol and to transmit extended data in accordance with a second protocol, comprising a first transmission means for transmitting said first data over time slots within time divisional multiple access frames; and second transmitting means for transmitting said extended data, wherein a second bandwidth for transmission of said extended data is divided into time slots substantially similar to said time slots for said first data, wherein said first transmission means include signalling means arranged to communicate via signalling channels and said signalling means arranged to establish data calls for said second transmitting means.